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NASA solves foam riddle

CAPE CANAVERAL, FLORIDA — More than a year after the Columbia tragedy, NASA has learned how and why the piece of foam insulation that doomed the shuttle broke off from the fuel tank at liftoff.

NASA's top spaceflight official, Bill Readdy, said Friday that extensive testing determined that air liquefied by the supercold fuel in the tank almost certainly seeped into a crack or void in the foam, or collected around bolts and nuts beneath it. The trapped air expanded as the shuttle rose, and blew off a suitcase-size chunk of foam.

Rather than peeling off, as NASA had assumed, the foam was pushed off with explosive force, Readdy said. The space agency also had assumed the foam would fall down along the tank and miss the shuttle, but it shot toward Columbia, and the left wing rammed into it.

"That is really the root cause that we've been able to discover here," Readdy said.